Magnetic resonance imaging

ABSTRACT

Improved magnetic resonance imaging systems, methods and software are described including a low field strength main magnet, a gradient coil assembly, an RF coil system, and a control system configured for the acquisition and processing of magnetic resonance imaging data from a patient while utilizing a sparse sampling imaging technique.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/630,890, filed Jun. 22, 2017, entitled “MAGNETIC RESONANCE IMAGING,”which claims the benefit of priority under 35 U.S.C. 119 of U.S.Provisional Application No. 62/353,538, filed Jun. 22, 2016, entitled“MAGNETIC RESONANCE IMAGING,” the disclosures of each are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The subject matter described herein relates to systems, methods andcomputer software for magnetic resonance imaging and various diagnosticand interventional applications associated therewith.

BACKGROUND

Magnetic resonance imaging (MRI), or nuclear magnetic resonance imaging,is a noninvasive imaging technique that uses the interaction betweenradio frequency pulses, a strong magnetic field (modified with weakgradient fields applied across it to localize and encode or decodephases and frequencies) and body tissue to obtain projections, spectralsignals, and images of planes or volumes from within a patient's body.Magnetic resonance imaging is particularly helpful in the imaging ofsoft tissues and may be used for the diagnosis of disease. Real-time orcine MRI may be used for the diagnosis of medical conditions requiringthe imaging of moving structures within a patient. Real-time MRI mayalso be used in conjunction with interventional procedures, such asradiation therapy or image guided surgery, and also in planning for suchprocedures.

SUMMARY

Magnetic resonance imaging systems, methods and software are disclosed.Some implementations may be used in conjunction with a main magnethaving a low field strength, a gradient coil assembly, an RF coilsystem, and a control system configured for the acquisition andprocessing of magnetic resonance imaging data from a human patient whileutilizing a sparse sampling imaging technique without parallel imaging.

In some variations, the field strength of the main magnet is less than1.0 Tesla and in others the field strength is approximately 0.35 T.

In some implementations, the control system of the MRI may be configuredto utilize low gradient field strengths (e.g., below 20 mT/m), toutilize large flip angles (e.g., greater than 40 degrees), to utilize RFbandwidths to maintain chemical shift and magnetic susceptibilityartifacts to less than one millimeter (e.g., RF bandwidths less than1800 Hz), to utilize a gradient slew rate above 75 mT/m/ms, and/or toemploy pulse sequences that do not require dephasing or spoiler pulses.In some implementations, the RF coil system may not include a surfacecoil.

The control system of the magnetic resonance imaging system may also beconfigured to produce cine MRI (e.g., of least 4 frames per second).

In another implementation, the magnetic resonance imaging system may beintegrated with a radiation therapy device for radiation treatment of ahuman patient and the control system may be further configured toutilize cine MRI to track the locations of tissues in the human patient.The radiation therapy device may be a linear accelerator having anenergy in the range of, for example, 4-6 MV. The radiation therapydevice may also be a proton therapy system, heavy ion therapy system, ora radioisotope therapy system.

The magnetic resonance imaging system may also comprise a split/openbore magnet and be configured to allow for surgical intervention in thegap of the split magnet, for example, with a robotic surgical deviceintegrated into the system. Similarly, the gradient coil assembly may bea split gradient coil assembly. The main magnet may be a superconductingmagnet, a non-superconducting magnet, or a resistive magnet. The mainmagnet may be powered by a battery system.

Implementations of the current subject matter can include, but are notlimited to, methods consistent with the descriptions provided herein aswell as articles and computer program products that comprise a tangiblyembodied machine-readable medium operable to cause one or more machines(e.g., computers, etc.) to result in operations implementing one or moreof the described features. Similarly, computer systems are alsocontemplated that may include one or more processors and one or morememories coupled to the one or more processors. A memory, which caninclude a computer-readable storage medium, may include, encode, store,or the like, one or more programs that cause one or more processors toperform one or more of the operations described herein. Computerimplemented methods consistent with one or more implementations of thecurrent subject matter can be implemented by one or more data processorsresiding in a single computing system or across multiple computingsystems. Such multiple computing systems can be connected and canexchange data and/or commands or other instructions or the like via oneor more connections, including but not limited to a connection over anetwork (e.g., the internet, a wireless wide area network, a local areanetwork, a wide area network, a wired network, or the like), via adirect connection between one or more of the multiple computing systems,etc.

The details of one or more variations of the subject matter describedherein are set forth in the accompanying drawings and the descriptionbelow. Other features and advantages of the subject matter describedherein will be apparent from the description and drawings, and from theclaims. While certain features of the currently disclosed subject matterare described for illustrative purposes in relation to particularimplementations, it should be readily understood that such features arenot intended to be limiting. The claims that follow this disclosure areintended to define the scope of the protected subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, show certain aspects of the subject matterdisclosed herein and, together with the description, help explain someof the principles associated with the disclosed implementations. In thedrawings,

FIG. 1 is a diagram illustrating a simplified perspective view of anexemplary magnetic resonance imaging system in accordance with certainaspects of the present disclosure.

FIG. 2 is a diagram illustrating a simplified perspective view of anexemplary magnetic resonance imaging system incorporating an exemplaryinterventional device in accordance with certain aspects of the presentdisclosure.

FIG. 3 is a simplified diagram for an exemplary method of real-timeMRI-guided radiation therapy in accordance with certain aspects of thepresent disclosure.

DETAILED DESCRIPTION

The present disclosure describes systems, methods and computer softwareallowing for, among other things, high-quality magnetic resonanceimaging with limited magnetic susceptibility distortions and chemicalshift artifacts resulting in submillimeter spatial accuracy, high framerate cine capability with an appropriate specific absorption rate (SAR),and the ability to support real-time 2-D and volumetric MRI-guideddiagnostic and interventional applications.

FIG. 1 illustrates one implementation of a magnetic resonance imagingsystem (MRI) 100 consistent with certain aspects of the presentdisclosure. In FIG. 1 , the MRI 100 includes a main magnet 102, agradient coil assembly 104 and an RF coil system 106. Within MRI 100 isa patient couch 108 on which a human patient 110 may lie. MRI 100 alsoincludes a control system 112, discussed in detail below.

The main magnet 102 of MRI 100 may be a cylindrical split or open boremagnet separated by buttresses 114, with a gap 116 as shown in FIG. 1 ,a closed-bore cylindrical configuration, a C-shaped configuration, adipolar magnet, or the like. Main magnet 102 may be comprised of anumber of magnet types, including electromagnets, permanent magnets,superconducting magnets, or combinations thereof. For example, onecombination or “hybrid” magnet may include permanent magnets andelectromagnets. Main magnet 102 may be configured for any commonly usedfield strength, but is preferably configured for a low field strength.When the term low field strength is used herein, it refers to a fieldstrength of less than 1.0 Tesla. In particular implementations of thepresent disclosure, the field strength of main magnet 102 may beconfigured to be in the range of 0.1 to 0.5 Tesla, or configured to beapproximately 0.35 Tesla. The system may be designed to use resistive orpermanent magnets, or a combination thereof, for example, when the fieldstrength of the main magnet is less than approximately 0.2 Tesla. In oneimplementation, a system utilizing resistive magnet(s) may be powered bya direct-current battery system, for example, a lithium ion system suchas, or similar to, a Tesla Powerwall.

Gradient coil assembly 104 contains the coils necessary to add smallvarying magnetic fields on top of main magnet 102's field to allow forspatial encoding of the imaging data. Gradient coil assembly 104 may bea continuous cylindrical assembly, a split gradient coil assembly asshown in FIG. 1 , or other designs as may be necessary for theparticular MRI configuration utilized.

RF coil system 106 is responsible for exciting the spins of hydrogenprotons within patient 110 and for receiving subsequent signals emittedfrom patient 110. RF coil system 106 thus includes an RF transmitterportion and an RF receive portion. The implementation in FIG. 1 includesa singular body coil performing both the RF transmit and RFfunctionalities. RF coil system 106 may alternatively divide transmitand receive functionalities between a body coil and a surface coil, ormay provide both transmit and receive functionalities within a surfacecoil. The RF coil system 106 depicted in the implementation of FIG. 1has a continuous cylindrical form but could also be designed in a splitmanner, so that gap 116 would be open from the patient to the outer edgeof main magnet 102.

Control system 112 is configured for the acquisition and processing ofmagnetic resonance imaging data from patient 110, including imagereconstruction. Control system 112 may contain numerous subsystems, forexample, those which control operation of the gradient coil assembly104, the RF coil system 106, portions of those systems themselves, andthose that process data received from RF coil system 106 and performimage reconstruction.

In one advantageous implementation, control system 112 is configured toutilize a sparse sampling imaging technique without parallel imaging.When the term sparse sampling imaging technique is used herein it refersto image acquisition and reconstruction techniques where only a portionof frequency space is measured (for the purposes of the presentdisclosure, 50% or less of the frequency information used to reconstructan image using standard back-projection methods), and the imagereconstruction is performed by optimization of the reconstructed imageto be consistent with a priori knowledge of the imaged subject whilealso generally satisfying consistency between the frequency informationof the reconstructed image and the measured frequency information.Sparse sampling imaging techniques thus include techniques such ascompressed sensing and the volumetric imaging technique disclosed inU.S. Patent Application No. 62/353,530, filed concurrently herewith andassigned to ViewRay Technologies, Inc.

Parallel imaging techniques are commonly used in magnetic resonanceimaging, especially with cine MRI, to shorten the time required for dataacquisition. Parallel imaging methods use knowledge of the spatialdistribution of signals received by multiple RF detectors (such as asurface coil having an array of these “elements”) to replace some of thetime-consuming phase-encoding steps in the MRI process. In this manner,signal is received from multiple coil elements “in parallel,” and thesampling of fewer portions in k-space along readout trajectories (i.e.,fewer phase encodings) is compensated for by the duplicity of data fromall coil elements.

However, certain implementations of the present disclosure contemplatedata acquisition and processing without utilizing parallel imagingtechniques. In such cases where the present disclosure refers tomagnetic resonance data acquisition and processing “without parallelimaging” it contemplates systems, methods and computer software designedto incorporate a small amount parallel imaging (perhaps in an attempt toavoid infringement), but not enough to create a perceptively significantincrease in signal-to-noise ratio, all other things being constant.

In some advantageous implementations, the MRI and control system 112 maybe configured to utilize low gradient field strengths, for example below20 mT/m or, in other cases, below 12 mT/m. In addition, someadvantageous implementations may utilize a relatively high gradient slewrate or rise time, such as a slew rate above 75 mT/m/ms. Control system112 may also be advantageously configured to utilize large flip angles,for example, greater than 40 degrees. In addition, control system 112may be advantageously configured to employ pulse sequences that do notrequire dephasing pulses (it is contemplated that such pulse sequenceshave no dephasing pulses, or have only a small number of dephasing orspoiling pulses such that there is no significant increase in dataacquisition time from the standpoint of patient throughput).

In some implementations, control system 112 may be configured to utilizeRF bandwidths to maintain chemical shift and magnetic susceptibilityartifacts below one millimeter or even below one half of a millimeter.As an example, control system 112 may be configured for an RF bandwidthless than 1800 Hz. An evaluation of potential worst-case artifacts dueto magnetic susceptibility and chemical shift can be evaluated. Forexample, for a worst case of, e.g., 8 ppm perturbation observed in humansusceptibility assessments, formula [1] below can be used to estimatemagnetic susceptibility artifacts.

$\begin{matrix}{{\delta_{ms}\lbrack{mm}\rbrack} = {{{mag}.{suscept}.\lbrack{ppm}\rbrack} \times \frac{B_{o}\lbrack T\rbrack}{G_{e}\left\lbrack {T/{mm}} \right\rbrack}}} & \lbrack 1\rbrack\end{matrix}$

Here δ_(ms)[mm] is the spatial distortion in millimeters due to magneticsusceptibility artifacts due to a magnetic susceptibility inducedmagnetic field change, mag. suscept.[ppm] in parts per million of themain magnetic field strength, B_(o)[T], in Tesla, and where G_(e)[T/mm]is the gradient encoding strength in Tesla per millimeter.

And, formula [2] below may be used to estimate displacements due tochemical shift.

$\begin{matrix}{{\delta_{cs}\lbrack{mm}\rbrack} = {{3.5\lbrack{ppm}\rbrack} \times {{PixelSize}\lbrack{mm}\rbrack} \times \frac{f_{B_{o}}\lbrack{Hz}\rbrack}{{BW}\left\lbrack {{Hz}/{pixel}} \right\rbrack}}} & \lbrack 2\rbrack\end{matrix}$

Here δ_(cs)[mm] is the spatial distortion in millimeters due to chemicalshift artifacts, where 3.5 [ppm] is the relative parts per milliondifference in the Larmour frequency for Hydrogen bound to Oxygen (H—O)versus Carbon (C—H) for a Pixel or Voxel size, PixelSize, inmillimeters, and f_(Bo) is the Larmour frequency for Hydrogen in waterand BW[Hz/pixel] is the frequency bandwidth for a pixel or voxel inHertz per pixel or voxel.

A worst-case distortion can be taken as the sum of these two distortionsplus any residual distortions due to uncorrected gradient fieldnonlinearities.

In one particular implementation of the magnetic resonance imagingsystem 100 of the present disclosure, the main magnet 102 field strengthis approximately 0.35 Tesla and control system 112 is configured toutilize gradient field strengths below 12 mT/m, a gradient slew rateabove 75 mT/m/ms, flip angles greater than 40 degrees, RF bandwidthsless than 1800 Hz and pulse sequences that do not contain dephasingpulses. Control system 112 may also be configured to utilize a sparsesampling imaging technique without parallel imaging.

In another implementation of magnetic resonance imaging system 100, themain magnet 102 field strength is approximately 0.15 Tesla and controlsystem 112 is configured to utilize gradient field strengths below 10mT/m, a gradient slew rate above 75 mT/m/ms, flip angles greater than 60degrees, RF bandwidths less than 1000 Hz and pulse sequences that do notcontain dephasing pulses. In this implementation, control system 112 mayalso be configured to utilize a sparse sampling imaging techniquewithout parallel imaging.

As discussed further herein, certain implementations of the systems,methods and computer software of present disclosure can be beneficialfor cine planar, cine multi-planar, or real time volumetric or “4-D”(3-D spatial plus the time dimension) magnetic resonance imaging.Control system 112 may thus be configured to acquire and process data asnecessary to reconstruct images to create cine MRI, for example,enabling cine MRI of at least 4 frames per second while maintaining anacceptable specific absorption rate in patient 110.

Conventional wisdom is that a high main magnet field strength is alwayspreferred due to higher signal-to-noise ratio, with the desired fieldstrength being limited mainly by size and cost considerations. Throughhigher signal-to-noise, contrast, and resolution, a higher fieldstrength typically facilitates an improved ability for physicians tomake diagnoses based on the resulting images. Yet, implementations ofthe present disclosure utilizing low main magnet field strengths (e.g.,below 1.0 Tesla) result in high quality images and provide a number ofadditional benefits.

For example, implementations of the present disclosure can include RFbandwidths less than 1800 Hz, resulting in increased chemical shiftartifacts (i.e., where hydrogen atoms in different chemical environmentssuch as water and fat are partially shielded from the main magneticfield due to the difference in sharing of electrons involved in O—H andC—H chemical bonds, and hence have different nuclear magnetic resonancechemical shifts, appearing in different spatial locations when locatingsignals with frequency encoding). While high field systems will exhibitsignificant chemical shift artifacts, and require higher RF bandwidths(and their accompanying lower signal-to-noise ratios), the low fieldsystems disclosed herein can use lower RF bandwidths and maintain highspatial integrity.

In addition, high main magnetic field strength systems will exhibitsignificant magnetic susceptibility artifacts where the diamagnetic andparamagnetic (and in rare cases ferromagnetic) nature of the imagedsubject perturbs the magnetic field, leading to spatially distortedimages. Such issues in higher field systems might typically be addressedthrough an increase in gradient field strengths, but implementations ofthe present disclosure avoid the same level of artifacts and thus mayutilize lower gradient field strengths, resulting in improvedsignal-to-noise ratio and a lower specific absorption rate.

Moreover, the systems, methods and software discussed herein can beimplemented without parallel imaging, which would cause a decrease inthe signal-to-noise ratio of the resulting images that would increasewith the speed of the imaging. Instead, the sparse sampling techniquesdisclosed herein allow for high frame rate acquisition with a relativelyhigh signal-to-noise ratio that does not significantly decrease withimage acquisition speed, for example, through the use of a priori dataacquired before scanning, avoiding the use of “gridded” k-space data,and applying iterative optimization techniques. The use of phased arrayreceive coils may also be avoided in the absence of parallel imaging,thereby achieving high quality imaging with less complex technology.Fewer RF receive channels may be used, in fact, only a single RF receivechannel may be employed, along with a less expensive spectrometer.

Certain implementations of the present disclosure can also be employedwithout surface coils in contact with the patient. Instead, imaging maybe performed with merely a body coil integrated into the bore of the MRIthat contains both the transmit and receive coils.

In addition, simultaneous multiple slice imaging techniques may bebeneficially employed, where multiple imaging slices or sub-volumes maybe simultaneously excited and simultaneously read out. Oneimplementation of simultaneous multiple slice excitation can summultiple RF waveforms with different phase modulation functionsresulting in a multiband pulse that can excite desired slices in thepresence of a common slice selective gradient.

Furthermore, implementations of the present disclosure may utilizerelatively high flip angles, which, at higher main magnet fieldstrengths, would cause excessive patient heating. The higher flip anglesin implementations of the present disclosure will result in improvedimage contrast and signal-to-noise ratios.

Additionally, the low main magnet field strength implementationsdiscussed herein will exhibit faster RF signal decay, allowing for pulsesequences that do not require dephasing pulses (with the attendantadvantage of a lower specific absorption rate).

The low main magnet field strength of certain implementations of thepresent disclosure also allows for lower frequency RF excitation pulsesand thus decreased heating of the patient tissues by those pulses.

Further still, the well-controlled specific absorption rates exhibitedby implementations of the present disclosure provide the ability toacquire and process data at a speed sufficient for high frame rate cineMRI.

With the numerous above described advantages, implementations of thepresent disclosure are well-suited for high quality cine MRI having anacceptable patient specific absorption rate. These implementations alsocontrol magnetic susceptibility and chemical shift artifacts so as toprovide high spatial integrity, which can be critical in certaindiagnostic and interventional applications.

Implementations of the present disclosure can be beneficial in numerousapplications for diagnostic cine MRI, examples include anatomiclocalizers, repeated rapid imaging for localization and the study ofmovement (e.g., phonation), imaging freely moving subjects (e.g., fetalMRI), cardiac imaging, and the like.

Implementations of the present disclosure can also be beneficial ininterventional applications, which also benefit from the advantages ofhigh spatial integrity and controlled specific absorption rate. Examplesof interventional applications include angioplasty, stent delivery,thrombolysis, aneurysm repair, vertebroplasty, fibroid embolization, andmany other applications where fluoroscopy is currently used (and wherethe use of cine MRI will decrease radiation dose to the patient).

Implementations of the present disclosure may also be used for imageguided surgery, and may provide real-time intraprocedural guidance inmultiple orthogonal planes, imaging feedback regarding instrumentposition, guidance and/or warning systems and the like. An open bore MRIimplementation, similar to that depicted in FIG. 1 (but with a split RFcoil system 106) can be particularly beneficial for such interventionalprocedures. MRI 100 may thus be configured to allow for surgicalintervention in the gap of a split magnet and may further include arobotic surgical device integrated with the system.

Yet another advantage of the low field strength attendant to certainimplementations of the present disclosure is the decreased magneticforces that will be exerted on any interventional equipment employed inconjunction with MRI 100 such as robotic surgery equipment, biopsyinstrumentation, cryogenic ablation units, brachytherapy equipment,radiation therapy equipment, and the like.

In one implementation of magnetic resonance imaging system 100, incombination with interventional equipment (e.g., radiation therapyequipment such as a linac), a low field strength, non-superconductingmagnet is utilized, for example, a resistive magnet, a permanent magnet,or hybrid magnet.

Another beneficial application of certain implementations of the presentdisclosure is in the field of image guided radiotherapy. Radiotherapyapplications will also benefit from the present disclosure's ability toprovide high frame rate cine MRI with high spatial integrity, both ofwhich are key to accurately tracking a target being treated and to avoidhitting patient critical structures with significant amounts of ionizingradiation.

FIG. 2 illustrates MRI 100 further configured to integrate a radiationtherapy device to treat patient 110. In one implementation, MRI 100 mayinclude a gantry 202 positioned in gap 116 of an open bore MRI. Gantry202 can incorporate radiation therapy device 204, configured to direct aradiation therapy beam 206 toward patient 110. In one particularimplementation, radiation therapy device 204 may be a linear acceleratorhaving an energy in the range of 4-6 MV and, as depicted, the componentsof the linear accelerator may be divided into separate shieldingcontainers 208 spaced about gantry 202. These linac components may thenbe connected by RF waveguides 210. While FIG. 2 depicts a particularradiation therapy device arrangement, the present disclosurecontemplates the integration of any type of radiation therapy systemsuch as proton therapy, heavy ion therapy, radioisotope therapy, etc.

As noted above, control system 112 of magnetic resonance imaging system100 may be configured for cine MRI and further configured to utilizecine MRI to track the locations of tissues in the human patient 110.

An additional benefit of implementations of the present disclosureutilizing a main magnet 102 with a low field strength is a decrease indistortions of the delivered ionizing radiation dose distribution inpatient 110 caused by the magnetic Lorenz force acting on the transportof secondary electrons (and positrons). The Lorenz force exerted by ahigher field main magnet would overpower the scattering power of theelectrons (and positrons), and cause them to spiral off their naturalcourse, trapping them at low density interfaces—potentially resulting inunintended and harmful dose concentrations in the patient.

An exemplary method for real-time image guided radiotherapy, consistentwith implementations of the present disclosure, is illustrated in FIG. 3. At 302, magnetic resonance imaging data may be acquired from a humanpatient 110 through magnetic resonance imaging system 100 having asuperconducting main magnet with low field strength, a gradient coilassembly 104, and an RF coil system 106, where the acquisition utilizesa sparse sampling imaging technique without parallel imaging. At 304,the magnetic resonance imaging data is processed. At 306, radiationtherapy is administered to human patient 110. At 308, the magneticresonance imaging data is utilized to track the locations of tissue(s)in the patient 110. And, at 310, the administration of radiation therapymay be altered based on the tracking of the location of tissue(s) inpatient 110. In altering therapy, actions such as stopping the therapy,reoptimizing the therapy, adjusting the radiation therapy beam and thelike are contemplated. The exemplary method illustrated in FIG. 3 mayalso incorporate any or all of the characteristics described above(e.g., low gradient field strengths, large flip angles, RF bandwidths tomaintain spatial integrity, particular pulse sequences, etc.).

When the present disclosure indicates that the magnetic resonanceimaging system is configured to operate in a particular manner, it meansthat such system is setup and intended to be operated in that manner,regardless of whether it may also be configured to utilize pulsesequence(s) or configurations that do not operate in the mannerdescribed or claimed herein.

The present disclosure contemplates that the calculations disclosed inthe embodiments herein may be performed in a number of ways, applyingthe same concepts taught herein, and that such calculations areequivalent to the embodiments disclosed.

One or more aspects or features of the subject matter described hereincan be realized in digital electronic circuitry, integrated circuitry,specially designed application specific integrated circuits (ASICs),field programmable gate arrays (FPGAs) computer hardware, firmware,software, and/or combinations thereof. These various aspects or featurescan include implementation in one or more computer programs that areexecutable and/or interpretable on a programmable system including atleast one programmable processor, which can be special or generalpurpose, coupled to receive data and instructions from, and to transmitdata and instructions to, a storage system, at least one input device,and at least one output device. The programmable system or computingsystem may include clients and servers. A client and server aregenerally remote from each other and typically interact through acommunication network. The relationship of client and server arises byvirtue of computer programs running on the respective computers andhaving a client-server relationship to each other.

These computer programs, which can also be referred to programs,software, software applications, applications, components, or code,include machine instructions for a programmable processor, and can beimplemented in a high-level procedural language, an object-orientedprogramming language, a functional programming language, a logicalprogramming language, and/or in assembly/machine language. As usedherein, the term “machine-readable medium” (or “computer readablemedium”) refers to any computer program product, apparatus and/ordevice, such as for example magnetic discs, optical disks, memory, andProgrammable Logic Devices (PLDs), used to provide machine instructionsand/or data to a programmable processor, including a machine-readablemedium that receives machine instructions as a machine-readable signal.The term “machine-readable signal” (or “computer readable signal”)refers to any signal used to provide machine instructions and/or data toa programmable processor. The machine-readable medium can store suchmachine instructions non-transitorily, such as for example as would anon-transient solid-state memory or a magnetic hard drive or anyequivalent storage medium. The machine-readable medium can alternativelyor additionally store such machine instructions in a transient manner,such as for example as would a processor cache or other random accessmemory associated with one or more physical processor cores.

To provide for interaction with a user, one or more aspects or featuresof the subject matter described herein can be implemented on a computerhaving a display device, such as for example a cathode ray tube (CRT) ora liquid crystal display (LCD) or a light emitting diode (LED) monitorfor displaying information to the user and a keyboard and a pointingdevice, such as for example a mouse or a trackball, by which the usermay provide input to the computer. Other kinds of devices can be used toprovide for interaction with a user as well. For example, feedbackprovided to the user can be any form of sensory feedback, such as forexample visual feedback, auditory feedback, or tactile feedback; andinput from the user may be received in any form, including, but notlimited to, acoustic, speech, or tactile input. Other possible inputdevices include, but are not limited to, touch screens or othertouch-sensitive devices such as single or multi-point resistive orcapacitive trackpads, voice recognition hardware and software, opticalscanners, optical pointers, digital image capture devices and associatedinterpretation software, and the like.

In the descriptions above and in the claims, phrases such as “at leastone of” or “one or more of” may occur followed by a conjunctive list ofelements or features. The term “and/or” may also occur in a list of twoor more elements or features. Unless otherwise implicitly or explicitlycontradicted by the context in which it used, such a phrase is intendedto mean any of the listed elements or features individually or any ofthe recited elements or features in combination with any of the otherrecited elements or features. For example, the phrases “at least one ofA and B;” “one or more of A and B;” and “A and/or B” are each intendedto mean “A alone, B alone, or A and B together.” A similarinterpretation is also intended for lists including three or more items.For example, the phrases “at least one of A, B, and C;” “one or more ofA, B, and C;” and “A, B, and/or C” are each intended to mean “A alone, Balone, C alone, A and B together, A and C together, B and C together, orA and B and C together.” Use of the term “based on,” above and in theclaims is intended to mean, “based at least in part on,” such that anunrecited feature or element is also permissible.

The subject matter described herein can be embodied in systems,apparatus, methods, computer programs and/or articles depending on thedesired configuration. Any methods or the logic flows depicted in theaccompanying figures and/or described herein do not necessarily requirethe particular order shown, or sequential order, to achieve desirableresults. The implementations set forth in the foregoing description donot represent all implementations consistent with the subject matterdescribed herein. Instead, they are merely some examples consistent withaspects related to the described subject matter. Although a fewvariations have been described in detail above, other modifications oradditions are possible. In particular, further features and/orvariations can be provided in addition to those set forth herein. Theimplementations described above can be directed to various combinationsand subcombinations of the disclosed features and/or combinations andsubcombinations of further features noted above. Furthermore, abovedescribed advantages are not intended to limit the application of anyissued claims to processes and structures accomplishing any or all ofthe advantages.

Additionally, section headings shall not limit or characterize theinvention(s) set out in any claims that may issue from this disclosure.Specifically, and by way of example, although the headings refer to a“Technical Field,” such claims should not be limited by the languagechosen under this heading to describe the so-called technical field.Further, the description of a technology in the “Background” is not tobe construed as an admission that technology is prior art to anyinvention(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the invention(s) set forth in issuedclaims. Furthermore, any reference to this disclosure in general or useof the word “invention” in the singular is not intended to imply anylimitation on the scope of the claims set forth below. Multipleinventions may be set forth according to the limitations of the multipleclaims issuing from this disclosure, and such claims accordingly definethe invention(s), and their equivalents, that are protected thereby.

What is claimed is:
 1. A magnetic resonance imaging system (MRI) comprising: a main magnet having a field strength less than 1.0 Tesla; a gradient coil assembly; an RF coil system; and a control system configured for acquisition and processing of magnetic resonance imaging data from a human patient and configured to utilize a sparse sampling imaging technique without parallel imaging and to employ simultaneous multiple slice imaging techniques.
 2. The magnetic resonance imaging system of claim 1, wherein the multiple slice imaging techniques comprise simultaneously exciting and simultaneously reading out a plurality of imaging slices or sub-volumes.
 3. The magnetic resonance imaging system of claim 2, the control system configured to generate a common slice selective gradient and to generate a multiband pulse to excite the plurality of imaging slices or sub-volumes in the presence of the common slice selective gradient.
 4. The magnetic resonance imaging system of claim 1, the multiple slice imaging techniques comprising generating a multiband pulse by summing multiple RF waveforms with different phase modulation functions.
 5. The magnetic resonance imaging system of claim 1, wherein the control system is further configured for an RF bandwidth to be less than 1800 Hz.
 6. The magnetic resonance imaging system of claim 1 wherein the control system is configured to employ pulse sequences that do not require dephasing pulses.
 7. A computer program product comprising a non-transient, machine-readable medium storing instructions which, when executed by at least one programmable processor, cause the at least one programmable processor to perform operations comprising: acquiring magnetic resonance imaging data from a human patient through a magnetic resonance imaging system (MRI) having a main magnet with low field strength, a gradient coil assembly and an RF coil system, the acquiring utilizing a sparse sampling imaging technique without parallel imaging and employing simultaneous multiple slice imaging techniques; and processing the magnetic resonance imaging data, the processing including reconstructing images of the human patient.
 8. The computer program product of claim 7, the operations further comprising exciting and simultaneously reading out a plurality of imaging slices or sub-volumes.
 9. The computer program product of claim 8, the operations further comprising: generating a common slice selective gradient; and generating a multiband pulse to excite the plurality of imaging slices or sub-volumes in the presence of the common slice selective gradient.
 10. The computer program product of claim 7, the operations further comprising generating a multiband pulse by summing multiple RF waveforms with different phase modulation functions.
 11. The computer program product of claim 7, wherein the control system is further configured for an RF bandwidth to be less than 1800 Hz.
 12. The computer program product of claim 7, wherein the acquiring employs pulse sequences that do not require dephasing pulses. 